Optocoupler systems are useful for many different applications wherein electrical isolation is needed between a first circuit and a second circuit. One exemplary application is to use an optocoupler to electrically isolate a user interface (e.g., a logic interface) from a high voltage signal.
Optocoupler systems include a first circuit and a second circuit that are electrically isolated from each other. The first circuit includes a light emitting diode (LED) that is coupled to a LED current source. The first circuit is optically coupled to a second circuit. The second circuit includes a photodiode (PD). For example, the LED emits light, which impinges on the photodiode, causing a current through the photodiode (e.g., a photodiode current). The second circuit also includes a transimpedance amplifier circuit that is coupled to the photodiode to generate an output voltage signal that is based on the photodiode current. The second circuit also includes a current source that generates a reference current. Typically, the photodiode current is compared with the reference signal, and this comparison is utilized to generate the output voltage signal.
An optocoupler system includes three primary components: 1) buffer, 2) isolation, and 3) detector. The buffer provides a constant current to drive a light source (e.g., a light emitting diode (LED)). This current is referred to as the LED current and caused the LED to light up. The light passes through the isolation that can be for example a transparent substance. It is noted that the light undergoes a certain amount of attenuation before receiving the detector. The detector converts the received light into corresponding electrical signals (e.g., a current signal and a voltage signal). The detector compares the voltage signal corresponding to the received light to a reference signal and generates an output signal (e.g., a “1” or “0”) that is based on the comparison.
An important component in optocoupler systems is the current source used to drive the light emitting diode. One desired operation characteristic of a current source is the provision of a constant current signal. Unfortunately, the power supply may not be constant. For example, in a system with a 5V power supply, the actual supply voltage may be 10% above or below the 5V at any time. In other words, the power supply voltage can vary from about 4.5V to 5.5V. This variation in power supply voltage is also referred to as “power supply swing.” Power supply swing introduces issues and design concerns in designing the current source.
One such design concern is that when the power supply swings, the current generated by a current source that is coupled to the power supply can vary widely due to channel length modulation (CLM). For-example, in the above example, the variation of the current provided by the current source can vary by more than ten percentage points and in some cases can vary by a few tens of percentage points. As can be appreciated, a current source with wide variation in output current is not desired in any application, especially the optocoupler.
Consequently, current variation due to channel length modulation (CLM) and power supply swing poses significant challenges for the design and construction of the current source. One prior art approach is to use a cascode current source. Unfortunately, a cascade current source has a large threshold voltage, which in addition to low power supply voltages, can cause the current source to fail under certain conditions. Others have proposed increasing the channel length of the current source. However, this attempt to address the channel length modulation problem also encounters headroom issues or requires the increase of the overall size of the transistor (e.g., the width of the device) used to implement the current source.
Based on the foregoing, there remains a need for a channel-length modulation (CLM) compensation method and apparatus that overcomes the disadvantages set forth previously.